Problem:
Impossible to change stastus of order which has applicated coupoun who has conditions to:
- Logged user
- Specific products
- Specific product categories

Cause:
Opencart allways check conditions by active sessions(products in cart), even when administrator updates order status in administration.

Best fix:
Just install this extension :)

Workaround (if you dont want to install anything):
When you update order status open frontend part in same browser and fullify coupon conditions (do not confirm order).

Enjoy